Three dimensional nand memory

ABSTRACT

A monolithic, three dimensional NAND string includes a first memory cell located over a second memory cell, a select transistor, a first word line of the first memory cell, a second word line of the second memory cell, a bit line, a source line, and a select gate line of the select transistor. The first and the second word lines are not parallel to the bit line, and the first and the second word lines extend parallel to at least one of the source line and the select gate line.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to Mokhlesi et al., U.S. application Ser.No. __/______, titled “THREE DIMENSIONAL NAND MEMORY” (Attorney DocketNo. 035905/0145), Mokhlesi et al., U.S. application Ser. No. __/______,titled “METHOD OF MAKING THREE DIMENSIONAL NAND MEMORY” (Attorney DocketNo. 035905/0146), Mokhlesi et al., U.S. application Ser. No. __/______,titled “METHOD OF MAKING THREE DIMENSIONAL NAND MEMORY” (Attorney DocketNo. 035905/0148), Mokhlesi et al., U.S. application Ser. No. __/______,titled “THREE DIMENSIONAL NAND MEMORY” (Attorney Docket No.035905/0149), and Mokhlesi et al., U.S. application Ser. No. __/______,titled “METHOD OF MAKING THREE DIMENSIONAL NAND MEMORY” (Attorney DocketNo. 035905/0150), each filed on the same day herewith, and each herebyincorporated by reference in its entirety.

BACKGROUND

The present invention relates generally to the field of semiconductordevices and specifically to three dimensional NAND strings and otherthree dimensional devices.

Three dimensional vertical NAND strings are disclosed in an article byT. Endoh, et. al., titled “Novel Ultra High Density Memory With AStacked-Surrounding Gate Transistor (S-SGT) Structured Cell”, IEDM Proc.(2001) 33-36. However, this NAND string provides only one bit per cell.Furthermore, the active regions of the NAND string is formed by arelatively difficult and time consuming process involving repeatedformation of sidewall spacers and etching of a portion of the substrate,which results in a roughly conical active region shape.

SUMMARY

According to one embodiment of the invention a monolithic, threedimensional NAND string comprises a first memory cell located over asecond memory cell, a select transistor, a first word line of the firstmemory cell, a second word line of the second memory cell, a bit line, asource line, and a select gate line of the select transistor. The firstand the second word lines are not parallel to the bit line, and thefirst and the second word lines extend parallel to at least one of thesource line and the select gate line.

According to another embodiment of the invention a monolithic, threedimensional NAND string comprises a first memory cell located over asecond memory cell, a first word line of the first memory cell, and asecond word line of the second memory cell. The first word line extendsin first direction, and the second word line extends in a seconddirection different from the first direction.

According to another embodiment of the invention a NAND array comprisesan array of vertical NAND strings, a plurality of word lines, aplurality of bit lines, and a plurality of source lines. The bit linesare not parallel to the word lines, the word lines are not parallel tothe source lines, and the source lines are not parallel to the bitlines.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 2A, 3A, 4A, 5A, 6A, 7A, 8A, 9A, 10A, 11A, 12A, 13A, 14A and15A are top views of steps of making a device according to the firstembodiment of the invention.

FIGS. 1B, 2B, 3B, 4B, 5B, 6B, 6C, 7B, 7C, 8B, 8C, 9B, 9C, 10B, 10C, 11B,11C, 12B, 12C, 13B, 13C, 14B, 14C, 15B and 15C are side cross sectionalviews of steps of making a device according to the first embodiment ofthe invention. FIG. 6D is a three dimensional view of an in-processdevice shown in FIG. 6A.

FIG. 16 illustrates a side cross sectional view along the bit linedirection of a completed vertical NAND string of the first embodiment ofthe invention.

FIGS. 17A and 17B illustrate side cross sectional views of portions ofthe access transistor of NAND string according to second and thirdembodiments of the invention.

FIGS. 18A and 19 illustrate a circuit schematics of the NAND string ofthe embodiments of the invention. FIG. 18B illustrates a portion of thecircuit schematic of FIG. 18A, but with the source lines, select linesand word lines removed for clarity.

DETAILED DESCRIPTION

Embodiments of the present invention will be described below withreference to the accompanying drawings. It should be understood that thefollowing description is intended to describe exemplary embodiments ofthe invention, and not to limit the invention.

The embodiments of the invention provide a monolithic, three dimensionalarray of memory devices, such as an array of vertical NAND strings. TheNAND strings are vertically oriented, such that at least one memory cellis located over another memory cell. The array allows vertical scalingof NAND devices to provide a higher density of memory cells per unitarea of silicon or other semiconductor material. This nonvolatile memorypreferably contains two charge trapping memory cells, such as SONOScells, per 4F² in each memory level. Therefore, a four memory cell levelconfiguration will have 0.5F² area per cell or 0.5F² binary bits percell. The array may have two or more memory cell levels, such as two toeight levels. Thus, an N memory cell level configuration will have4F²/2N area per cell. If desired, the select transistors for each NANDstrings may also be monolithically integrated into each NAND stringabove and/or below the memory cells.

A monolithic three dimensional memory array is one in which multiplememory levels are formed above a single substrate, such as asemiconductor wafer, with no intervening substrates. The term“monolithic” means that layers of each level of the array are directlydeposited on the layers of each underlying level of the array. Incontrast, two dimensional arrays may be formed separately and thenpackaged together to form a non-monolithic memory device. For example,non-monolithic stacked memories have been constructed by forming memorylevels on separate substrates and adhering the memory levels atop eachother, as in Leedy, U.S. Pat. No. 5,915,167, titled “Three DimensionalStructure Memory.” The substrates may be thinned or removed from thememory levels before bonding, but as the memory levels are initiallyformed over separate substrates, such memories are not true monolithicthree dimensional memory arrays.

The preferred programming and erase method of the NAND string is viaFowler-Nordheim (“FN”) tunneling. Multilevel cell (“MLC”) operation ofthe NAND string of the multiple VT states type or of the Saifon/mirrorbit type is also possible.

Thus, the array contains two bits per 4F² in each memory level andprovides further scaling by vertically integrating multiple memorylevels. Each charge trapping memory cell can be operated in a binaryfashion providing large margins and high performance. Furtherefficiencies are provided by the fact that select transistors can alsobe vertically integrated and one or possibly both select transistors maybe completely omitted. The vertical integration of select transistorseliminates any break in regular line and space patterning of masks usedfor each device level. There is no break in the continuity of regularand completely periodic lines and spaces within the entire memory array,allowing for small device features with a narrow pitch formed bylithography. In contrast to prior art two dimensional planar NANDdevices, no extra space needs to be created for end of NAND string linesand spaces.

Alternative embodiments include configurations with select gates formedin trenches in the silicon wafer or other substrate, configurations withno select gate (i.e., no select gate lines and no select transistors),configurations with only select gate drain, configurations with onlyselect gate source, and configurations with both select gates.Orientation of the select gate lines with respect to the orientation ofthe source lines, bit lines and word lines can be varied in variousconfigurations. Even non-right angle orientation of various lines withrespect to one another is possible, as will be described below. In someembodiments, source lines may be replaced by a common source regionextending in both dimensions of the plane of the substrate and providinghigher current sinking capability at the expense of inability to selectindividual source line voltages. The orientation of the memory levelswith respect to one another can be varied also. For example, each memorylevel can have word lines that are oriented in perpendicular directionsto the level above and to the level below.

FIGS. 1A and 1B illustrate a first step in the method of making a NANDstring according to a first embodiment of the invention. FIG. 1A is atop view and FIG. 1B is a side cross sectional view along line A-A inFIG. 1A which extends parallel to the word lines. FIG. 1B illustrates ap-type silicon substrate 1 containing an n-type silicon layer 3 adjacentto the surface. It should be noted that p-type and n-type regions may bereversed and that semiconductor materials other than silicon, such asgallium arsenide for example, may be used. The substrate 1 and layer 3preferably comprise single crystal silicon. Layer 3 may be formed by ablanket ion implantation or by epitaxially growing an n-type layer on ap-type substrate. The active regions 5 in substrate 1 and layer 3 areseparated from each other by insulating isolation regions 7. Anysuitable isolation regions 7 may be used, such as LOCOS silicon oxide orSTI oxide filled trenches. Preferably, the pn junction between substrate1 and layer 3 is located above the bottom of the isolation regions 7,such as above the STI trench bottom, to be able to drive each activedevice's voltage independent of the other devices. The STI isolationregions 7 may be formed by patterning and etching standard STI trenches,performing thermal or radical liner oxidation, depositing trench filloxide, and planarizing the fill oxide to the top of the silicon layer 3by any suitable planarization method, such as chemical-mechanicalpolishing (CMP).

FIGS. 2A and 2B illustrate a second step in the method of making theNAND string. FIG. 2A is a top view and FIG. 2B is a side cross sectionalview along line A-A in FIG. 2A which extends parallel to the word lines.As shown in FIG. 2B, a silicon layer 9 is epitaxially grown on theactive regions 5 exposed between the isolation regions 7. The activeregions 5 act as a seed for the epitaxial growth of layer 9. Therefore,the grain boundaries 11 in layer 9 are formed over the isolation regions7, while essentially single crystal silicon regions in layer 9 areformed over the active regions 5.

Layer 9 contains a p-type region 15 between n-type regions 13 and 17.Layer 9 may be doped in-situ during growth by changing the dopantconcentrations in the precursor gases. This forms the npn structure 13,15, 17 that will later define source/channel/drain regions of verticalsidewall MOS select transistors. Ion implantation or other forms ofdoping the various layers 13-17 are also possible but result in a morecomplex process flow. The n-type region 13 electrically and physicallycontacts the n-type active regions 5 in layer 3.

FIGS. 3A and 3B illustrate a third step in the method of making the NANDstring. FIG. 3A is a top view and FIG. 3B is a side cross sectional viewalong line A-A in FIG. 3A which extends parallel to the word lines. Asshown in FIG. 3B, the epitaxial layer 9 is planarized by any suitableplanarization method, such as CMP, to provide a planar upper surface.

FIGS. 4A and 4B illustrate a fourth step in the method of making theNAND string. FIG. 4A is a top view and FIG. 4B is a side cross sectionalview along line A-A in FIG. 4A which extends parallel to the word lines.The epitaxial layer 9 is patterned into strips 19. As used herein, theterm “strip” refers to a body that has a length which is much greaterthan its thickness or its width and which extends in one direction alongits length. The strips 19 in the first embodiment extend along the bitline direction, as will be explained in more detail below.

The strips 19 are formed by forming a mask over the layer 9, such as aphotolithographically patterned photoresist layer mask, and etching theunmasked portions of layer 9. As shown in FIGS. 5A and 5B, thepatterning of the strips is not necessarily self aligned to the activeregions S below. Preferably, but not necessarily, the strips 19 are notaligned to the active regions 5, such that the strips 19 extendlaterally past the active regions S and over the isolation regions 7, asshown in FIG. 5B and/or such that a portion of the active regions 5 areexposed below the strips 19, as shown in FIG. 5A.

FIGS. 5A and 5B illustrate a fifth step in the method of making the NANDstring. FIG. 5A is a top view and FIG. 5B is a side cross sectional viewalong line A-A in FIG. 4A which extends parallel to the word lines.

As shown in FIGS. 5A and 5B, an insulating layer, such as silicon oxideand/or another insulating layer 21 is deposited between the strips andplanarized with the top surface of the strips 19. The insulating layer21 may be planarized by CMP or other planarization methods, such as etchback.

FIGS. 6A to 6D illustrate a sixth step in the method of making the NANDstring. FIG. 6A is a top view and FIG. 6B is a side cross sectional viewalong line A-A in FIG. 6A which extends parallel to the word lines. FIG.6C is a side cross sectional view along line B-B in FIG. 6A whichextends parallel to the bit lines. FIG. 6D is a three dimensional viewof the in process device shown in FIGS. 6A-6C.

The strips 19 and portions of the insulating layer 21 between the strips19 are patterned into strips 23 which extend parallel to the word linedirection and perpendicular to strips 19. The strips 23 are formed byforming a mask over the strips 19 and insulating layer 21, such as aphotolithographically patterned photoresist layer mask, and etching theunmasked portions of strips 19 and layer 21.

The strips 23 consist of semiconductor pillars 25 separated fromadjacent pillars in the word line direction by the portions of theinsulating layer 21. Each pillar 25 is separated from adjacent pillarsin the bit line direction by the trenches 27 between pillars. Eachpillar 25 contains a p-type conductivity semiconductor region 15 locatedbetween n-type conductivity type semiconductor regions 13, 17 in thevertical direction (i.e., region 15 is above region 13 and below region17 with respect to substrate 1).

Preferably, each pillar 25 has a square or rectangular cross sectionwhen viewed from the top, as shown in FIG. 6A. Thus, each pillar 25preferably has four vertical sides.

FIGS. 7A to 7C illustrate a seventh step in the method of making theNAND string. FIG. 7A is a top view and FIG. 7B is a side cross sectionalview along line A-A in FIG. 7A which extends parallel to the word lines.FIG. 7C is a side cross sectional view along line B-B in FIG. 7A whichextends parallel to the bit lines.

As shown in FIG. 7C, a gate insulating layer 29 is formed in thetrenches 27 between the pillars 25 and over the top surfaces of thepillars 25. The gate insulating layer 29 may comprise silicon oxide,silicon nitride or any other suitable gate insulating layer material. Ifdesired, layer 29 may contain two or more sublayers having a differentcomposition.

A select gate layer is then deposited over the gate insulating layer 29.One or more of any suitable gate electrode materials may be used for theselect gate layer, such as polysilicon, silicide (titanium silicide,etc.), tungsten, aluminum or a combination of sublayers of thesematerials.

The select gate layer is then planarized with the top of the gateinsulating layer 29 by any suitable planarization method, such as CMP.The planarization leaves the select gates 31 located in portions of thetrenches 27 above the gate insulating layer 29, as shown in FIG. 7C.

FIGS. 8A to 8C illustrate an eighth step in the method of making theNAND string. FIG. 8A is a top view and FIG. 8B is a side cross sectionalview along line A-A in FIG. 8A which extends parallel to the word lines.FIG. 8C is a side cross sectional view along line B-B in FIG. 8A whichextends parallel to the bit lines.

As shown in FIG. 8C, the select gates 31 are partially etched back sothat the top of the select gates are located below the tops of thepillars 25. The gates 31 may be etched back using a selective etch whichetches the gate material selectively over the gate insulating layer 29material.

FIGS. 9A to 9C illustrate a ninth step in the method of making the NANDstring. FIG. 9A is a top view and FIG. 9B is a side cross sectional viewalong line A-A in FIG. 9A which extends parallel to the word lines. FIG.9C is a side cross sectional view along line B-B in FIG. 9A whichextends parallel to the bit lines.

An insulating cap layer is deposited over the recessed select gates 31and over the gate insulating layer 29. Preferably, the cap layercomprises the same material as the gate insulating layer 29, such assilicon oxide. The cap layer is then planarized, such as CMP planarized,to fill the trenches located above the select gates 31 and to forminsulating caps 33 located above each select gate 31. The cap 33electrically isolates the select gates from the NAND string memory cellsthat will be formed above. During the planarization of the cap layer,the portions of the gate insulating layer 29 located above thesemiconductor pillars 25 are also removed to expose the top region 17 ofthe pillars 25.

As shown in FIG. 9A, the select gates 31 comprise portions of selectgate lines which extend in the word line direction. Thus, the selectgate lines comprise strip shaped lines located in trenches 27 (whichwere shown in FIG. 6A). Each select gate 31 acts as a gate electrode fortwo adjacent select transistors 35 to the left and right of the gate 31in FIG. 7C.

Thus, the ninth step completes the bottom select transistor 35 for theNAND string. Each select field effect transistor 35 comprises the pillar25 active region, in which region 15 acts as a channel and regions 13and 17 as “source” and “drain” regions, a select gate 31 which acts asthe gate electrode of the transistor, and the gate insulating layer 29located between the select gate 31 and the pillar 25. Since each pillar25 is located between two different select gates 31, the left and rightside of each pillar 25 can be considered as a separate select transistor35 for the same NAND string to be formed above the pillar 25.

FIGS. 10A to 10C illustrate a tenth step in the method of making theNAND string. FIG. 10A is a top view and FIG. 10B is a side crosssectional view along line A-A in FIG. 10A which extends parallel to theword lines. FIG. 10C is a side cross sectional view along line B-B inFIG. 10A which extends parallel to the bit lines,

FIGS. 10A-10C illustrate the first step in the formation of the memorycells above the select transistors 35. First, the silicon surface of theexposed pillars 25 is preferably cleaned after the CMP step in FIG. 9C.For example, the top surface of each silicon pillar may be treated bythermal or radical oxidation (i.e., to form a silicon oxide layer on thetop of the pillars) followed by a wet, gentle oxide etch in order toremove the oxide layer along with the damage incurred during CMP and/ordry etches, to prepare the silicon surface for the growth of nextepitaxial layer. Such damage may impact quality of the subsequentepitaxial layer growth.

Then, as shown in FIGS. 10A-10C, the next epitaxial layer 109 is grownon the completed select gate transistors 35. The subsequent steps offorming the first NAND memory cell are similar to the method steps shownin FIGS. 2-9, except that a charge storage region is formed instead of agate insulating layer 29.

As shown in FIGS. 10B and 10C, the silicon layer 109 is epitxially grownon the pillar active regions 25 exposed between the isolation regionsformed by insulating layers 21, 29 and 33. For example, plasma assistedepitaxy (i.e., PECVD) may be used to grow the silicon layer 109 at lowertemperatures, such as at 700° C. and below, for example at around 650°C. While higher temperature growth processes may be used, the lowtemperature PECVD process allows the use of lower thermal budget metalsand dielectrics (i.e., metals and dielectrics that cannot withstandtemperatures above 700° C.) and provide for more controlled junctiondepths and channel lengths.

The exposed box shape upper surface of pillar active regions 25 act as aseed for the epitaxial growth of layer 109. Therefore, the grainboundaries 111 in layer 109 are formed over the isolation regions, whileessentially single crystal silicon regions in layer 109 are formed overthe active regions 25. The grain growth of layer 109 mushrooms out fromthe seeds 25 below and forms grain boundaries 111 where the grains meeteach other during the epitaxy process. Thus, the position of the grainboundaries 111 will be where random grains meet and the grain boundaries111 will generally not be as smooth and predictable as schematicallyillustrated in FIGS. 10A-10C. However, the grain boundaries are locatedin regions that will be etched away during subsequent steps. Thus, ahigh level of smoothness and predictability is not required.

Layer 109 contains a p-type region 115 located between n-type regions113 and 117 in the vertical direction. Layer 109 may be doped in-situduring growth by changing the dopant concentrations in the precursorgases. This forms the npn structure 113, 115, 117 that will later definesource/channel/drain regions of charge trapping MOS memory devices(i.e., the NAND memory cells). Ion implantation or other forms of dopingthe various layers 113-117 are also possible but result in a morecomplex process flow. The n-type regions 113 electrically and physicallycontact the n-type active regions 17 in pillar 25.

FIGS. 11A to 11C illustrate an eleventh step in the method of making theNAND string. FIG. 11A is a top view and FIG. 11B is a side crosssectional view along line A-A in FIG. 11A which extends parallel to theword lines. FIG. 11C is a side cross sectional view along line B-B inFIG. 11A which extends parallel to the bit lines.

As shown in FIGS. 11B and 11C, the epitaxial layer 109 is planarized byany suitable planarization method, such as CMP, to provide a planarupper surface.

FIGS. 12A to 12C illustrate a twelfth step in the method of making theNAND string. FIG. 12A is a top view and FIG. 12B is a side crosssectional view along line A-A in FIG. 12A which extends parallel to theword lines. FIG. 12C is a side cross sectional view along line B-B inFIG. 12A which extends parallel to the bit lines.

The epitaxial layer 109 is patterned into strips 119. As used herein,the term “strip” refers to a body that has a length which is muchgreater than its thickness or its width and which extends in onedirection along its length. The strips 119 in the first embodimentextend along the bit line direction, as will be explained in more detailbelow.

The strips 119 are formed by forming a mask over the layer 109, such asa photolithographically patterned photoresist layer mask, and etchingthe unmasked portions of layer 109. As shown in FIGS. 12A-12C, thepatterning of the strips is not necessarily self aligned to the pillaractive regions 25 below. Preferably, but not necessarily, the strips 119are not aligned to the active regions 25, such that the strips 119extend laterally past the active regions 25 and over the isolationregions formed by layers 21, 29 and 33 which surround the pillars 25, asshown in FIGS. 12B and 12C and/or such that a portion of the activeregions 25 are exposed below the strips 119, as shown in FIG. 12A.

FIGS. 13A to 13C illustrate a thirteenth step in the method of makingthe NAND string. FIG. 13A is a top view and FIG. 13B is a side crosssectional view along line A-A in FIG. 13A which extends parallel to theword lines. FIG. 13C is a side cross sectional view along line B-B inFIG. 13A which extends parallel to the bit lines.

As shown in FIGS. 13A-13B, an insulating layer, such as silicon oxideand/or another insulating layer 121 is deposited between the strips 119adjacent to exposed lateral sides of the strips 119. Layer 121 is thenplanarized with the top surface of the strips 119. The insulating layer121 may be planarized by CMP or other planarization methods, such asetch back.

FIGS. 14A to 14C illustrate a fourteenth step in the method of makingthe NAND string. FIG. 14A is a top view and FIG. 14B is a side crosssectional view along line A-A in FIG. 14A which extends parallel to theword lines. FIG. 14C is a side cross sectional view along line B-B inFIG. 14A which extends parallel to the bit lines

The strips 119 and portions of the insulating layer 121 between thestrips 119 are patterned into strips 123 which extend parallel to theword line direction and perpendicular to strips 119. The strips 123 areformed by forming a mask over the strips 119 and insulating layer 121,such as a photolithographically patterned photoresist layer mask, andetching the unmasked portions of strips 119 and layer 121.

The strips 123 consist of semiconductor pillars 125 separated fromadjacent pillars in the word line direction by the portions of theinsulating layer 121. Each pillar 125 is separated from adjacent pillarsin the bit line direction by the trenches 127 between pillars. Eachpillar 125 contains a p-type conductivity semiconductor region 115located between n-type conductivity type semiconductor regions 113, 117in the vertical direction (i.e., region 115 is above region 113 andbelow region 117 with respect to substrate 1).

Preferably, each pillar 125 has a square or rectangular cross sectionwhen viewed from the top, as shown in FIG. 14A. Thus, each pillar 125preferably has four vertical sides.

FIGS. 15A to 15C illustrate a fifteenth step in the method of making theNAND string. FIG. 15A is a top view and FIG. 15B is a side crosssectional view along line A-A in FIG. 15A which extends parallel to theword lines. FIG. 15C is a side cross sectional view along line B-B inFIG. 15A which extends parallel to the bit lines.

As shown in FIGS. 15A-15C, a charge storage region is formed between thestrips 123. The charge storage region may comprise a dielectric isolatedfloating gate or a dielectric charge storage material. For example, toform a dielectric isolated floating gate, a polysilicon layer isdeposited between two insulating layers, such as silicon oxide tunnelingand blocking layers. For example, sidewall spacer formed floating gatesmay be used. The additional space taken by spacer floating gate may berecouped by utilizing multilevel cell (MLC) programming for thesedevices.

To form a dielectric charge storage region, a charge storage dielectriclayer is deposited between tunneling and blocking dielectric (i.e.,insulating) layers. For example, the charge storage dielectric layer maycomprise a silicon nitride layer while the tunneling and blocking layersmay comprise silicon oxide layers to form an “ONO” charge storage regionof a “SONOS” type device. Preferably, the tunneling dielectric layer isthinner than the blocking dielectric layer.

However, materials other than silicon nitride and silicon oxide may beused instead. For example, TANOS type devices may be used. As disclosedin U.S. Pat. No. 6,858,899, which is incorporated herein by reference inits entirety, high dielectric constant insulating materials, such asmaterials having a dielectric constant of above 3.9, may be used for thetunneling and/or the blocking dielectric layer instead of silicon oxide.These materials include metal oxide layers, such as aluminum oxide,tantalum oxide, yttrium oxide, calcium oxide, magnesium oxide orzirconium oxide. The charge storage dielectric may alternativelycomprise a silicon oxynitride layer in which a portion of nitrogen inthe silicon nitride layer is substituted with oxygen. Alternatively, ametal oxide layer, such as tantalum oxide, zirconium oxide or hafniumoxide, may be used as the charge storage dielectric.

In the following discussion, the ONO charge storage region will bedescribed. However, it should be understood that a floating gate chargestorage region or other dielectric charge storage material combinationsmay be used instead.

As shown in FIGS. 15A and 15C, a tunneling dielectric layer 128, acharge storage dielectric layer 129 and a blocking dielectric layer 130are formed in that order in the trenches 127 between the pillars 125(i.e., adjacent to exposed sides of the pillars) and over the topsurfaces of the pillars 125. The tunneling and blocking dielectrics maycomprise silicon oxide while the charge storage dielectric may comprisesilicon nitride.

A control gate layer is then deposited over the dielectric layers128-130. One or more of any suitable gate electrode materials may beused for the control gate layer, such as polysilicon, silicide (titaniumsilicide, etc.), tungsten, aluminum or a combination of sublayers ofthese materials.

The control gate layer is then planarized with the top of the tunnelinglayer 128 by any suitable planarization method, such as CMP. Theplanarization leaves control gates 131 located in portions of thetrenches 127 above the dielectric layers 128-130.

The control gates 131 are partially etched back so that the top of thegates are located below the tops of the pillars 125. The gates 131 maybe etched back using a selective etch which etches the gate materialselectively over the ONO dielectric layers 128-130.

An insulating cap layer is then deposited over the recessed controlgates 131 and over the ONO dielectrics. Preferably, the cap layercomprises the same material as the blocking dielectric 130, such assilicon oxide. The cap layer is then planarized, such as CMP planarized,to fill the trenches located above the control gates 131 and to forminsulating caps 133 located above each control gate 131. The cap 133electrically isolates the control gates from additional NAND stringmemory cells that will be formed above. During the planarization of thecap layer, the portions of the ONO dielectric layers 128-130 locatedabove the semiconductor pillars 125 are also removed to expose the topregion 117 of the pillars 125.

As shown in FIG. 15A, the control gates 131 comprise portions of wordlines which extend below the caps 133 in the word line direction. Thus,the word gate lines comprise strip shaped lines located in trenches 127.Each control gate 131 acts as a gate electrode for two adjacent memorycells 135 to the left and right of the gate 131 in FIG. 15C.

This completes the bottom memory cell 135 for the NAND string. Eachmemory cell 135 comprises the pillar 125 active region, in which region115 acts as a channel and regions 113 and 117 as “source” and “drain”regions, a control gate/word line 131 which acts as the gate electrodeof the transistor, and the charge storage region, such as the ONOdielectric layers 128-130 located between the control gate 131 and thepillar 125. Since each pillar 125 is located between two differentcontrol gates 131, the left and right side of each pillar 125 can beconsidered as a memory cell.

FIG. 16 illustrates a side cross sectional view along the bit linedirection of a completed vertical NAND string. A second level of memorycells 235 which are identical to the first memory cells 135 is formed onthe first memory cells 135 by repeating the process steps describedabove with respect to FIGS. 10-15 to form a multilevel vertical NANDstring. If desired, additional levels of memory cells, such as two tosix levels of memory cells may be formed over the first level of memorycells 135 by repeating the process steps described above. A plurality ofbit lines 137 are then formed above the upper most level of memorycells. The bit lines 137 contacts the pillar active regions of the upperlevel of memory cells. For example, the single bit line 137 shown inFIG. 16 extends perpendicular to the word lines 131, 231 of the memorycells. However, the bit lines 137 may extend in other directions as willbe described in more detail below.

Furthermore, if desired, an upper select transistor may be above theupper level of the memory cells below the bit lines 137 using the samemethod as the lower select gate transistor 35. The upper select gatetransistor is formed in addition to or instead of the lower select gatetransistor 35.

Thus, FIG. 16 illustrates the vertical NAND string 100 which is formedvertically over the substrate. One memory cell 235 is located in anupper device level and another memory cell 135 is located in a lowerdevice level located over the substrate and below the first device level235. Since active regions 125 and 225 are grown in different epitaxialgrowth steps, a defined boundary exists between the semiconductor activeregions 125 and 225. The boundary may comprise a dislocation, a grainboundary or a lateral offset of the pillar 225 with respect to pillar125 at the boundary. In contrast, the prior art vertical NAND stringdescribed in T. Endoh, et. al., titled “Novel Ultra High Density MemoryWith A Stacked-Surrounding Gate Transistor (S-SGT) Structured Cell”,IEDM Proc. (2001) 33-36 is formed by plural etching steps of the sameregion of the substrate.

Furthermore, the pillar shaped active regions of the vertical NANDstring memory cells made by the method described above have a square orrectangular cross section when it is viewed from the top. This providesseparate faces for each word line in each cell and allows the two bitper cell configuration. The pillar shaped active regions are formed bypatterning the active layer into strips and then patterning the stripsinto pillars. In contrast, the active regions of Endoh et al. have acircular cross section when viewed from the top. The active regions aresurrounded by a surround gate for a one bit per cell configuration.

The semiconductor active region 25 of the select transistor 35 comprisesa pillar. The semiconductor active region 125 of the lower memory cellcomprises a pillar which is not aligned with the semiconductor activeregion 25 of the select transistor 35. In the non-limiting embodimentshown in FIG. 16, the active region 125 extends laterally in at leastone direction past the semiconductor active region 25 of the selecttransistor 35. Likewise, the pillar active region 225 extends laterallyin at least one direction past the pillar active region 125 of cell 135,such that the pillar 125 is not aligned with the pillar 225.

The semiconductor active region of the memory cell 135 is a pillar 125comprising a first conductivity type semiconductor region 115 locatedbetween second conductivity type semiconductor regions 113, 117. Thesemiconductor active region of memory cell 235 is a pillar 225comprising a first conductivity type semiconductor region 215 locatedbetween second conductivity type semiconductor regions 213, 217. Thesecond conductivity type semiconductor region 213 in pillar 225 contactssecond conductivity type semiconductor region 117 in pillar 125.

As shown in FIG. 16, in the lower memory cell 135, a first chargestorage dielectric 129A is located adjacent to one side of the firstconductivity type semiconductor region 115 in the pillar 125 and a firstcontrol gate 131A is located adjacent to the first charge storagedielectric 129A. A second charge storage dielectric 129B is locatedadjacent to the opposite side of first conductivity type semiconductorregion 115 in the pillar 125, and a second control gate 131B is locatedadjacent to the second charge storage dielectric 129B. A similarconfiguration is present in the upper memory cell 235, where the twocharge storage dielectrics and two control gates are located on oppositesides of region 215 in pillar 225.

FIGS. 17A and 17B illustrate side cross sectional views of portions ofthe select transistor of NAND string according to alternative second andthird embodiments of the invention.

FIG. 17A a side cross sectional view along the word line direction ofthe second embodiment in which the lower select transistor 35 isomitted. In this case, the bottom memory cell level is formed over thesubstrate 1.

FIG. 17B a side cross sectional view along the bit line direction of thethird embodiment in which the select gates 31 of the select transistors35 are formed in trenches in the substrate 1. In this embodiment, ap-type substrate 1 contains an n-p-n structure 13, 15, 17 formed by ionimplantation of n-type ions into the substrate 1 to leave a p-typeregions of the substrate 15 between the implanted n-type regions 13 and17. Alternatively, regions 13-17 may be formed by epitaxial layer growthand in-situ doping during growth. Then, trenches are formed byphotolithography and etching through the npn structure to the p-typeportions of the substrate 1. The trenches are filled with an insulatingmaterial 20, such as silicon oxide. The insulating material 20 is thenpatterned by photolithography and etching to form additional trenches inthe material 20. These additional trenches are filled with the selectgate material which is then planarized to form the select gates 31. Ifthe select transistor 35 is omitted, then the lowest memory cell 135 maybe formed in the trench instead.

In an alternative fourth embodiment, the pillar active regions 25, 125,etc. of the select transistors and/or of the memory cells are formed inpolycrystalline semiconductor material 9, 109, etc. Thus, rather thanforming epitaxial semiconductor layers 9, 109, etc. on the underlyingpillars, an amorphous, microcrystalline or polycrystalline semiconductorlayer, such as a silicon layer, is formed on the underlying pillars.This amorphous, microcrystalline or polycrystalline semiconductor layeris then recrystallized to form a large grain polycrystallinesemiconductor material layer, such as a large grain polysilicon layer.The recrystallization may be conducted by any suitable annealing method,such as thermal annealing in a furnace, laser annealing and/or flashlamp annealing. This recrystallized layer is then patterned into pillaractive regions 25, 125, etc., as described above. The use of lowtemperature deposited and recrystallized polysilicon allows the activeregions to be formed over metal wiring or electrodes which cannotwithstand high temperatures.

Thus, the semiconductor active region of an upper memory cell can beformed epitaxially on a semiconductor active region of the underlyingmemory cell or the semiconductor active regions of one or more firstmemory cells may be formed in recrystallized polysilicon. The activeregions of the lowest level of the memory cells is formed epitaxially orby recrystallization on a semiconductor active region of the selecttransistor. The active regions of the select transistors are formedepitaxially or by recrystallization over the substrate.

The memory array's size in the lateral dimensions is limited by the RCtime constants of the word lines, select gate lines, source lines, andthe bit lines. The NAND strings are oriented vertically, and the channelregions (P-regions 115 the NMOS memory embodiment shown) are notgrounded. Thus, care has to be taken to manage this floating bodypotential. The inversion layer on the opposite (non-selected) side canbe created and utilized to help anchor the potential of the floatingP-type bodies during various operations such as read, program, and/orerase.

Highly doped N and P regions with abrupt junctions may also be used suchthat floating bodies can be more strongly coupled to one another throughthinner depletion regions. Another way of dictating the floating bodypotentials is through their junction leakages.

Furthermore, boosting for program inhibit should be much more efficient.However, the silicon pillar active regions can be driven as opposed tobeing boosted, allowing for more abrupt junctions.

Each memory cell and select transistor level is completely self alignedto itself. In other words, no separate alignment steps between devicelevels is required. Furthermore, each device level only requires twolithography steps—the first step to form the first strips 119 and thesecond step to form strips 123. The remaining features in each devicelevel are formed by layer deposition and planarization. Thus, at leastone region or layer of the NAND string 100, and preferably pluralregions or layers are planarized by CMP and/or other methods. Forexample, for cell 135 the semiconductor active region 125 is planarizedwhen it is in the form of epitaxial layer 109, as shown in FIGS. 11B and11C), the insulating layer 121 which insulates the NAND string 100 fromat least one other adjacent NAND string is planarized as shown in FIG.13B, and the charge storage dielectric 129, the control gate 131 and thecap layer 133 are planarized as shown in FIGS. 15B and 15C. Thus, atleast five layers (not counting the tunneling and blocking dielectrics)are planarized by CMP in each cell 135, 235, etc.

If desired, the silicon wafer substrate 1 may be rotated 45° during alllithography steps so that the wafer notch is not at the 12 o'clockposition but at the 1:30 position. In this case, then the vertical sidewall channels will be on a [100] crystallographic plane, providinghigher channel mobility.

Each device level is not self aligned to the level below it. However,this is of little consequence because the regions where the levels meetis intentionally designed to be the inactive source/drain regions of theNAND chain. The vertical dimensions of each level and the positions ofthe PN junctions in each level can be different from the other levelsbased on thermal budgets related to annealing of various levels. Lowtemperature (such as a temperature below 700° C.) semiconductorepitaxial growth, such as PECVD growth, and plasma oxidation may beemployed to minimize level to level variation. This also allows a singlehigh temperature anneal after the forming of all memory and select gatelevels. However, separate level by level annealing, or multiple annealsteps for each memory/select level may also be used. If desired, ananneal in a hydrogen ambient may also be conducted.

As noted above, the pillars are preferably rectangular or square whenviewed from the side. However, when the trench side walls are notvertical, the active layers, such as the select transistor pillar activeregion portions 5, will be in the form of truncated pyramids with largerrectangular or square bases than tops. Thus, certain amount ofmisalignment will not result in variation in contact areas of top of onesilicon pillar to the bottom of silicon pillar of the layer above it.

FIG. 18A illustrates a circuit schematic of an array of NAND stringsdescribed above. FIG. 18B illustrates a portion of the circuit schematicof FIG. 18A, but with the source lines, select lines and word linesremoved for clarity. FIGS. 18A and 18B illustrate the select transistors35, located on a substrate or in a trench in the substrate and at leasttwo levels of memory cells located vertically over the selecttransistors 35. Each NAND string is depicted as a single column, inwhich each level of memory cells is located over the underlying level ofmemory cells. For example, the middle vertical NAND string 100controlled by bit line 237 in column M includes the select transistor 35and four memory cells 135, 235, 345 and 445 in four levels. The selecttransistor 35 is connected to source line SL in row N+½. The selecttransistor 35 is controlled by select gate lines 31 in rows N and N+1.The lowest memory cell 135 is controlled by word lines 131 in rows N andN+1 in vertical level 1 (shown in FIG. 18A as WL (N+X row, Z level),such as WL (N, 1) for word line in row N, level 1). The other memorycells 235, 335 and 445 are controlled by word lines 231, 331 and 441 inrows N and N+1 in levels 2, 3 and 4, respectively. The upper memory cell445 is electrically connected to the bit line 237 in bit line column M.

Thus, each vertical NAND string includes the select transistor 35 andvertically arranged memory cells 135-445 located one over another. Theword lines 131 to 431 are not parallel to the bit line 237. For example,the word lines extend perpendicular to the bit line 237. However, theword lines 131-431 extend parallel to at least one of the source line239 and the select gate line 31, such as parallel to both the sourceline 239 and the select gate line 31.

In one alternative embodiment, the word lines in different verticallevels can extend in different directions from each other. For example,the word lines 131 in memory cell level one may extend in a differentdirection, such as in a perpendicular direction, from the word lines 231in memory cell level two. The word line direction may be alternatedbetween each memory cell level. For example, the word lines in levelsone and three may extend in one direction and the word lines in levelstwo and four may extend in a different direction. The word linedirections may differ by one to ninety degrees from each other. Thisconfiguration may reduce coupling between device levels by placingcharge storage locations adjacent to different faces of the pillaractive regions in adjacent memory cell levels (for example the charge isstored adjacent to north and south faces of the pillar in levels one andthree and adjacent to east and west faces in levels two and four).

In another alternative embodiment shown in FIG. 19, the bit lines, theword lines and the source lines are not parallel to each other. In otherwords, the bit lines 237 are not parallel to the word lines 131-431which are not parallel to the source lines 239 which are not parallel tothe bit lines. For example, as shown in FIG. 19, the word lines 131-431may extend perpendicular to the source lines 239, while the bit lines237 extend diagonally (i.e., at an angle of 1 to 89 degrees, such as 30to 60 degrees, for example 45 degrees) with respect to the word linesand the source lines. This allows simultaneous programming of differentmulti-state V_(T) levels to a group of memory cells on the same wordline by raising both the source line and bit line of each of the NANDstrings to provide a variety of effective programming/inhibit voltages.The current from each bit line sinks to an individually selected sourceline, thus decreasing the amount of current provided to a particularsource line. The diagonal bit lines of FIG. 19 may have a narrower pitchthan the bit lines shown in FIGS. 18A and 18B.

If desired, the arrangement may be varied such that the word and bitlines are perpendicular to each other and the source lines are diagonal.The source lines may be formed on top and the bit lines may be formed onthe bottom. This allows formation of metal and/or silicide rather thansemiconductor source lines, which leads to a decreased current crowdingdue to the lower resistivity source line material. If desired, all threetypes of lines may be non-perpendicular to each other and extenddiagonally with respect to each other. Preferably, the select lines areparallel to the word lines.

As shown in FIG. 19, each memory cell has a different associated wordline, bit line and source line combination that all other memory cellsin the array. For example, all memory cells in one row parallel to theword line direction are controlled by different bit lines and differentsource lines. The configuration of FIG. 19 allows each memory cell inthe array to be individually programmed (instead of programming eachadjacent pair of cells together) even when two adjacent cells share thesame word line, because these adjacent cells are connected to adifferent combination of bit lines and source lines from each other. Forexample, two adjacent cells in the same column parallel to one sourceline are controlled by a different bit line. Thus, two adjacent cells inthe same column are associated with the same word line and source linebut a different bit line. If desired, the select transistors 31 mayoptionally be omitted in the configuration of FIG. 19 due to the abilityto program each memory cell individually using bit line by bit linecontrol for programming of the cells. However, the progranirnngpreferably takes place level by level in each NAND string 200, withalternating levels being programmed sequentially.

In another alternative embodiment, the source lines 239 are be replacedby a common source region (source plane) extending in both dimensions ofthe plane of the substrate 1 (i.e., in the x-y plane). The common sourceregion may comprise a common conductive plate, such as a highly dopedsingle crystal or polycrystalline semiconductor, silicide and/or metalplate, which electrically contacts the pillar active regions 25 of allselect transistors 35 of the array. If the select transistors areomitted, then the source plate contacts the pillars 125 of the lowestmemory cell 135 level. The common source plate provides a higher currentsinking capability at the expense of loosing the ability to selectindividual source line voltages.

An alternative embodiment for MLC operation has both source lines andbit lines extending along the same direction to provide means to varythe entire NAND chain voltage on a bit line by bit line basis in orderto program cells that are being programmed to higher V_(T) states fasterthan cells that are being programmed to lower V_(T) states. The sourceand bit line voltages of cells that are being programmed to lower V_(T)states will be raised in order to retard programming of some thesecells, so that the entire set of states in a two or three dimensionalconfigurations will be programmed using fewer program pulses.

The foregoing description of the embodiments of the invention has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and modifications and variations are possible in light of theabove teaching or may be acquired from practice of the invention. Theembodiments were chosen and described in order to explain the principlesof the invention and as a practical application to enable one skilled inthe art to utilize the invention in various embodiments and with variousmodification are suited to the particular use contemplated. It isintended that the scope of the invention be defined by the claimsappended hereto and their equivalents.

1. A monolithic, three dimensional NAND string, comprising: a firstmemory cell located over a second memory cell; a select transistor; afirst word line of the first memory cell; a second word line of thesecond memory cell; a bit line; a source line; and a select gate line ofthe select transistor; wherein: the first and the second word lines arenot parallel to the bit line; and the first and the second word linesextend parallel to at least one of the source line and the select gateline.
 2. The NAND string of claim 1, wherein: the NAND string is formedvertically over a substrate; the select transistor is located on asubstrate or in a trench in the substrate; the first memory cell islocated in a first device level; and the second memory cell is locatedin a second device level located on the select transistor and below thefirst device level.
 3. The NAND string of claim 2, wherein: asemiconductor active region of the first memory cell is formedepitaxially on a semiconductor active region of the second memory cell;the semiconductor active region of the second memory cell is formedepitaxially on a semiconductor active region of the select transistor; afirst charge storage dielectric is located between the semiconductoractive region of the first memory cell and the first word line; and asecond charge storage dielectric is located between the semiconductoractive region of the second memory cell and the second word line.
 4. TheNAND string of claim 3, wherein: the semiconductor active region of thefirst memory cell comprises a first pillar comprising a firstconductivity type semiconductor region located between secondconductivity type semiconductor regions; the semiconductor active regionof the second memory cell comprises a second pillar comprising a firstconductivity type semiconductor region located between secondconductivity type semiconductor regions; the semiconductor active regionof the select transistor comprises a third pillar; one secondconductivity type semiconductor region in the first pillar contacts onesecond conductivity type semiconductor region in the second pillar; thefirst pillar is not aligned with the second pillar, such that the firstpillar extends laterally past the second pillar; and the second pillaris not aligned with the third pillar, such that the second pillarextends laterally past the third pillar.
 5. The NAND string of claim 1,wherein: the first and the second word lines extend perpendicular to thebit line; and the first and the second word lines extend parallel toboth the source line and the select gate line.
 6. The NAND string ofclaim 1, wherein: the first and the second word lines extendperpendicular to the bit line; and the first and the second word linesextend parallel to the source line.
 7. The NAND string of claim 1,wherein: the first and the second word lines extend perpendicular to thebit line; and the first and the second word lines extend parallel to theselect gate line.
 8. A monolithic, three dimensional NAND stringcomprising: a first memory cell located over a second memory cell; afirst word line of the first memory cell; and a second word line of thesecond memory cell; wherein: the first word line extends in firstdirection; and the second word line extends in a second directiondifferent from the first direction.
 9. The NAND string of claim 8,wherein: the NAND string is formed vertically over a substrate; a selecttransistor is located on a substrate or in a trench in the substrate;the first memory cell is located in a first device level; and the secondmemory cell is located in a second device level located on the selecttransistor and below the first device level.
 10. The NAND string ofclaim 9, wherein: a semiconductor active region of the first memory cellis formed epitaxially on a semiconductor active region of the secondmemory cell; the semiconductor active region of the second memory cellis formed epitaxially on a semiconductor active region of the selecttransistor; a first charge storage dielectric is located between thesemiconductor active region of the first memory cell and the first wordline; and a second charge storage dielectric is located between thesemiconductor active region of the second memory cell and the secondword line.
 11. The NAND string of claim 10, wherein: the semiconductoractive region of the first memory cell comprises a first pillarcomprising a first conductivity type semiconductor region locatedbetween second conductivity type semiconductor regions; thesemiconductor active region of the second memory cell comprises a secondpillar comprising a first conductivity type semiconductor region locatedbetween second conductivity type semiconductor regions; thesemiconductor active region of the select transistor comprises a thirdpillar; one second conductivity type semiconductor region in the firstpillar contacts one second conductivity type semiconductor region in thesecond pillar; the first pillar is not aligned with the second pillar,such that the first pillar extends laterally past the second pillar; andthe second pillar is not aligned with the third pillar, such that thesecond pillar extends laterally past the third pillar.
 12. The NANDstring of claim 8, wherein the first word line extends perpendicular tothe second word line.
 13. A NAND array, comprising: an array of verticalNAND strings; a plurality of word lines; a plurality of bit lines; and aplurality of source lines; wherein: the bit lines are not parallel tothe word lines; the word lines are not parallel to the source lines; andthe source lines are not parallel to the bit lines.
 14. The array ofclaim 13, wherein: the word lines extend perpendicular to the sourcelines; and the bit lines extend diagonally with respect to the wordlines and the source lines.
 15. The array of claim 13, wherein: the wordlines extend perpendicular to the bit lines; and the source lines extenddiagonally with respect to the word lines and the bit lines.
 16. Thearray of claim 13, wherein: each vertical NAND string comprises at leasttwo vertically arranged memory cells; and each memory cell of eachvertical NAND string has a different associated word line, bit line andsource line combination that all other memory cells in the array. 17.The array of claim 16, wherein each memory cell in the array is adaptedto be individually programmed.
 18. A NAND array, comprising: an array ofvertical NAND strings; a plurality of word lines; a plurality of bitlines; and a common source plane electrically connected each of thevertical NAND strings.
 19. The array of claim 18, wherein the commonsource plane comprises a conductive plate.
 20. The array of claim 19,wherein the conductive plate electrically contacts an active region of aselect transistor of each of the vertical NAND strings.